Patent GB-739501-A (GB)
Title: Improvements in or relating to fluid pressure relays
IPC: G05B7/04
Filed: 1953-09-23
Abstract: 739,501. Fluid-pressure servomotor-control systems. BAILEY METERS & CONTROLS, Ltd. Sept. 23, 1953 [Sept. 23, 1952], No. 26253/53. Class 135. A fluid-pressure relay comprises a pair of balance beams 10, 13, Fig. 2, to each of which at least one fluid pressure force is applied by means of expansible and contractible chambers A, B, and C, D acting on the respective beams, and a pair of co-operating elements, such as the baffle 21 and the nozzle 20, positioned by the respective beams to control the application of a fluid-pressure force to one of the beams following a change of position of the other beam so that the first mentioned beam takes up a position corresponding to that of the second mentioned beam. The invention is particularly applicable to fluid-pressure control systems in which a variable electrical, thermal, chemical or physical condition is controlled automatically or manually since it enables a control station for changing over from manual to automatic control or vice versa to be located at a point remote from the regulating unit and from the element detecting the variable condition. In such systems provided with adjustments for the control point and proportional band, these adjustments may be effected by the invention by means remote from the detector or transmitter of the variable condition. The relay 4, Fig. 1, is fed by a pipe 3 with an input pressure determined by a pilot valve 9 operated by a Bourdon tube 8 influenced by pressure variations in a pipe 2. The output pressure of the relay is fed by a pipe 5 to the actuating diaphragm 6 of a valve 7 for controlling the pressure in the conduit 2 or some other variable. Fig. 2 shows the relay 4 on a larger scale. The balance beams 10, 13 are pivoted by the respective blade springs 11, 14 and loaded by adjustable springs 15, 17, the beam 13 being supported in a substantially vertical position on a bracket 12 and carrying at its lower end the nozzle 20. The fluid pressure forces are applied to the beams 10, 13 by metallic bellows A, B, C, D, diaphragms or similar well-known devices. The bellows A and B are of equal area as also are the bellows C and D, but the bellows C and D are not necessarily of different area from the bellows A and B. The baffle 21 is carried on a baffle arm 37 attached to the beam 10. The input pressure from the pipe 3 may be applied to either of the bellows A B depending on the action to be given to the valve 7. As shown, it is applied to the chamber A and the chambers B and C are open to the atmosphere. Variable loading pressures may however be applied to any or all of three chambers A, B, C. The output pressure is supplied to the pipe 5 and chamber D by a pipe 34 from a pilot valve 33 comprising a piston valve 36 carried by a stem 32 and actuated through a yoke 31 from a spring- loaded bellows 28. The valve 36 is adapted to connect the pipe 34 to an air pressure supply port 35 or an exhaust port 35<SP>11</SP>. The bellows 28 is subject to the pressure in a chamber 26 fed with compressed air through a restricted orifice 25 provided with a normally retracted cleaning pin 38. The chamber 26 is connected to the nozzle 20 by a flexible pipe 22 so that relative movement between the nozzle 20 and the baffle 21 affects the pressure in the chamber 26 and therefore in the bellows 28. The proportional band is adjusted by adjustment of the. operative relation between the beam 10 and the baffle 21, i.e. the movement of the baffle that will be produced by a given movement of the beam 10. For this purpose the baffle arm 37 is pivoted at 47 to a link 41 in turn pivoted by a blade spring 42 to a toothed quadrant 43 pivoted about a fixed axis and adjusted by a hand-operated pinion 44. A stop 51, carried by the beam 10, co-operates with an arcuate slot 52 in the quadrant 43, the narrow end of the slot forming the narrow proportional band limit, i.e. the point where a relative small change in the angular position of the beam 10 is necessary to produce a given change in output pressure, and the wide end forming the wide proportional band limit. In the normal position of the beams, i.e. the beam 10 horizontal and the beam 13 vertical, the pivot 47 is coaxial with the fixed axis of the quadrant 43. Fig. 9 shows a system in which the regulating valve 7 is actually in the pipe 2 and a selector station 60 is provided to control the connections between the relay 4 and the valve 7. The input pressure in the pipe 3 is regulated by a mechanism as described in Specification 690,601 and this pressure is indicated at the station 60 through a pipe 3A and also at a local indicator 52 through a pipe 53. Change-over from automatic control is made by a knob 64. During hand-operation pressure is varied in the pipe 5 and also in the bellows C by a pressure-control device 69 adjusted by a knob 65. During automatic operation, the control point is adjusted by adjusting the pressure in the bellows B by a pressure control device 69<SP>1</SP> adjustable by a knob 66. Such control point pressure is indicated at the station 60 and on the indicator 52. The valves 1<SP>1</SP>, 2<SP>1</SP>, 31, 41, 51 for effecting the various control positions are operated by the knob 64. In this arrangement a throttle 68 between the bellows C and D is partly open so that the resultant action gives a floating control of high-sensitivity superimposed on a proportional control which may be of relatively low sensitivity.